NOTE: Colored sample tubes should not be used, as they may leak contaminant chemicals into the mixture. The microcentrifuge should
have a phenol‐free lid seal and built‐in vacuum delay to prevent bumping by allowing the rotor to achieve speed before applying
vacuum.NOTE: For urine samples, normalize the extraction volume for each sample to clinical creatinine levels (a measure of glomerular
filtration rate) or osmolality levels (a measure of the total concentration of all solutes, including salts and metabolites).
Concentrations of metabolites in urine vary drastically, e.g., influenced by the total volume of liquids a subject has consumed
in the hours before urine collection. In order to have a similar number of metabolites detected in urine, and to avoid saturating
the detector, the volume of urine used for metabolite extractions should be controlled for the total concentration of metabolites
prior to data acquisition. Creatinine or osmolality are regarded as good surrogate measures for this total urinary metabolite
concentration.

Support Protocol 1: Preparation of External and Internal Reference Standards for Quality Control